The present invention relates to a thermal insulation device for insulating the upper area of an annular space provided between the main vessel and the safety vessel of a fast neutron nuclear reactor.
In fast neutron nuclear reactors it is known that the reactor core is located in a vertically axed cylindrical vessel, called the main vessel, which is duplicated by an also cylindrical safety vessel, whose axis coincides with that of the main vessel. These vessels are sealed in their upper part by a normally concrete horizontal slab, which also supports the said vessels. Therefore the upper ends of the main vessel and the safety vessel are embedded in the slab. The main vessel also contains a appropriate volume of a liquid metal such as sodium ensuring the extraction of calories given off by the fission reaction of the fuel in the core. In operation the main vessel temperature is consequently relatively high. The space defined between the main vessel and the safety vessel is filled with an inert gas such as nitrogen, which is raised to a relatively high temperature due to its contact with the outer wall of the main vessel. However, the connecting areas of the vessels to the slab, as well as the parts of the vessels extending these areas below the slab constitute sensitive means for which it is desirable to limit to the greatest possible extent the thermal gradients prejudicial to their mechanical performance and, due to the fact that they are responsible for the suspension of the vessels, the safety of the reactor.
It is therefore considered necessary to insulate the upper area of the annular space provided between the vessels in order to keep the inert gas contained therein at a constant temperature which is significantly below that in the remainder of the annular space. It is also advantageous to provide a thermal insulation device permitting a relatively easy access to the insulated area, particularly for the purpose of inspecting welds within the vessel. The thermal insulation device which thermally insulates the upper area of the annular space between the vessels must also be designed so as to withstand the axial and transverse dimensional variations between the two vessels made inevitable by the temperature changes occurring as a result of the reactor operating conditions. This device must also be able to maintain its insulating capacity, even in the presence of aerosols of liquid cooling metal in the hypothesis of the leak thereof through the main vessel wall. Finally the thermal insulation device must be able to deform in the case of a mechanical or thermal incident in such a way that it does not puncture the vessels.
French patent application No. 74 29649 in the name of the Commissariat a l'Energie Atomique (corresponding to U.S. Pat. No. 4,050,988--Lemercier) already discloses a thermal insulaton device which meets the aforementioned requirements. This device is characterized in that it comprises a lower, generally cylindrical thermal insulation ring mounted beneath the slab and defining with the latter and the wall of the facing vessels the area to be insulated. This ring extends across the annular space in a horizontal plane and is supported by the slab via supporting ties. There is also an upper thermal insulation ring subdivided into two coaxial parts resting on the lower ring and respectively applied to the walls of the two vessels by a system of radially positioned spacing springs which bear against the two parts.
Satisfactory results are generally obtained with such a thermal insulation device. However, it has been found that the coaxial parts constituting the upper thermal insulation ring may seize up or jam after a certain period of use. Jamming of the parts bearing in each case against the vessels under the action of the spacing springs make the latter ineffective, which can lead to a loss of sealing of the device when the temperature variations of the vessels lead to an increase in the width of the annular space between the vessels.